JI-.VE28, 1895.] 



SCIENCE. 



719 



I taken as indicative of decided diflVrence of 

 opinion between these two geologists, l.nit 

 rather of a rapid change of opinion on the 

 part of the officei-s of the survey in conse- 

 quence of the recent paleontological deter- 

 minations of Hyatt, whose results were 

 probably not available at the time the earlier 

 folios went to press. The Mariposii formation 

 is of economic importance as that in which 

 occurs the zone of aui-iferous veining which 

 constitutes the famous ' Mother Lode." 



In a field so overburdened with igneous 

 rocks, contemporaneous and intrusive, 

 geologists will readily understand that 

 manj' problems arise which are not easily 

 answered by the most earnest efforts of the 

 field geologist. The lack of definite state- 

 ments as to the structural relations of the 

 various sedimentary and igneous forma- 

 tions indicates that these relations are ob- 

 scure and difficult to determine. Still, a 

 brief statement from Messrs. Turner and 

 Lindgren as to the interpretation of their 

 sti'uctural sections would have been a de- 

 sirable addition to the letter press, which is 

 limited strictly to historical, petrogi'aphic 

 and economic geology. For example : Are 

 the two belts of the Mariposa slates on the 

 Jackson sheet essentially synclinal troughs 

 with an anticline bringing up a belt of the 

 lower Calaveras between them ? If so, the 

 structure is comparatively simple, and the 

 great body of amphibolite schist, diabase 

 and porphyrite probably represents vol- 

 canic accumulations chiefly intermediate in 

 age between the Calaveras and the Mari- 

 posa, but perhaps passing up into the lat- 

 ter. Or is the region traversed by a great 

 system of longitudinal faults ? A discus- 

 sion of these and other tectonic questions 

 we may doubtless expect in more detailed 

 reports upon the geology of the region. 

 But something of the tectonic should find a 

 place in the folios to help out the sections. 

 While alluding to the igneous rocks it may 

 be well to mention that the user of the geo- 



logical map is handicapped by not having 

 the eflusive rocks discriminated from the 

 intrusive on the color scale. From tlie text 

 it is appan-nt that many of the igneous 

 rocks are clearly intrusive, wliile others are 

 eflusive. This discrimination should be 

 expressed graphically, as it is impossible to 

 understand the structure without keeping 

 it in mind. The doubtful rocks should be 

 grouped apart from those wliich are clearly 

 efi"usive or intrusive. An extra convention 

 or two to express doubt or ignorance on 

 particular points would greatly enhance the 

 scientific value of our geological maps. 



One of the most important features of the 

 SieiTa Nevada slope is the invasion of the 

 Calaveras and Mariposa formations by the 

 Sierra Nevada batholite. The relations of 

 the older rocks to this invading magma are 

 beautifully l)rought out by the careful map- 

 ping of Messi-s. Turner and Lindgren. 

 Petrogi-aphically, the rocks of this batholite 

 are chiefly of a type intermediate between 

 granite and diorite, and are therefore desig- 

 nated as granodiorite. Other important 

 facies of the same magma are gi-anite, gab- 

 bro and gabbro-diorite. These rocks ap- 

 pear as great intrusive areas in the midst of 

 the auriferous slates and establish pro- 

 nounced zones of contact metamorphism in 

 the latter. Putting the three geologic sheets 

 together, and bearing in mind the distribu- 

 tion of these same granitic rocks to the 

 eastward and southeastward of the area 

 mapped, it is diflicult to resist the sugges- 

 tion that these rocks underlie practically 

 the whole of the Sierra slope beneath the 

 rocks through which thej- project as isolated 

 masses. In other words, the mapping sug- 

 gests strongly that if the i>lane of truncation 

 efl'ected by erosion had been lower a much 

 larger proi)oi-tion of granite would have 

 been exposed, and if higher less. If this 

 suggestion be accepted it follows that the 

 Calaveras and Mariposa formations must 

 have reposed ujion the granodiorite magma 



